1. Field
The present disclosure relates to an electron emitting device that emits electrons in response to an applied voltage.
2. Description of the Related Art
A known example of an electron emitting device is one that utilizes field electron emission. In field electron emission, a voltage is applied between two electrodes to emit electrons. The application of the voltage causes a high electric field to be formed between the electrodes, and thereby electrons are emitted from one of the electrodes (emitter) due to a tunnel effect. Field electron emitting devices of a Spindt type, a carbon nanotube (CNT) type, and so forth, which have different emitter structures, are available.
There has been a demand for use of an electron emitting device in an air atmosphere. However, it is theoretically difficult to operate the above-described electron emitting device that utilizes field electron emission in an air atmosphere. The reason is as follows. A high electric field is necessary to realize field electron emission, and emitted electrons have high energy. If high-energy electrons collide with gas molecules in the air, the gas molecules are ionized. Positive ions produced by the ionization are accelerated, toward the surface of the device, by a high electric field formed near the device, collide with the device, and cause sputtering. The sputtering may break the electron emitting device. Further, in a case where high-energy electrons collide with oxygen molecules, ionization does not occur but ozone is produced. Ozone is very active, is harmful, and deteriorates various substances.
For the above-described reason, an electron emitting device that utilizes field electron emission is generally used while being sealed in a vacuum. In a case where electrons are to be taken from the vacuum, an electron transmission window that separates a vacuum layer and an air atmosphere from each other is set, and the electrons are transmitted from the vacuum layer into the air.
As other types of electron emitting devices, electron emitting devices of a metal insulator metal (MIM) type and a metal insulator semiconductor (MIS) type are available.
These are electron emitting devices of a surface emission type that accelerate electrons by utilizing a quantum size effect and an intense electric field inside the devices and emit the electrons from plane surfaces of the devices. These devices emit electrons that have been accelerated in an electron acceleration layer inside the devices, and thus it is not necessary to form an intense electric field outside the devices. Therefore, electron emitting devices of an MIM type and an MIS type may overcome disadvantages that electron emitting devices of a Spindt type, a CNT type, and a BN type may have, such as breakdown due to sputtering caused by ionization of gas molecules, and production of ozone.
Japanese Unexamined Patent Application Publication No. 2009-146891 (published on Jul. 2, 2009) discloses an electron emitting device including metal microparticles and insulating microparticles that have antioxidant properties. The electron emitting device described in the publication is capable of stably emitting electrons in an air atmosphere as well as in a vacuum, and does not produce harmful substances, such as ozone and NOx.
Regarding the above-described electron emitting devices of an MIM type and an MIS type, there is a need for increasing the size of the device so as to increase the area of a region from which electrons may be emitted. However, in the electron emitting devices of an MIM type and an MIS type according to the related art, it is difficult to evenly emit electrons from the devices if the area of the region from which electrons may be emitted is increased. This will be described below with reference to FIGS. 8A and 8B.
FIGS. 8A and 8B are diagrams illustrating a schematic configuration of an electron emitting device 110, which is a typical MIM-type electron emitting device according to the related art. FIG. 8A is a cross-sectional view, and FIG. 8B is a top view. The electron emitting device 110 includes a lower electrode 102, a surface electrode 103, and an electron acceleration layer 104. The electron acceleration layer 104 includes insulating microparticles and conductive microparticles that are dispersed among the insulating microparticles.
The easiness of emission of electrons in the electron emitting device 110 depends on many parameters, such as the thickness of the electron acceleration layer 104, the distribution of the conductive microparticles dispersed in the electron acceleration layer 104, and the thickness of the surface electrode 103. These parameters spatially vary to some extent in the process of manufacturing the electron emitting device 110. Spatial unevenness of the easiness of emission of electrons in the electron emitting device 110 occurs, in short, as a result of multiplying the variations of the above-mentioned parameters. FIG. 8B is a top view illustrating the electron emitting device 110 in a case where the easiness of emission of electrons is spatially uneven. The electron acceleration layer 104 is not illustrated in FIG. 8B. The surface electrode 103, which is a surface for emitting electrons in the electron emitting device 110, includes regions R1 and R2 from which electrons are easily emitted, and a region R3 from which electrons are not easily emitted. Thus, if a voltage is applied to the lower electrode 102 and the surface electrode 103 by a power supply 120, electrons are easily emitted from the regions R1 and R2. On the other hand, the amount of electrons emitted from the region R3 is smaller than that emitted from the regions R1 and R2.
In this way, the electron emitting device 110 includes regions from which electrons are easily emitted and a region from which electrons are not easily emitted, and accordingly the distribution of electrons emitted by the electron emitting device 110 is spatially uneven. The distribution of emitted electrons becomes more uneven as the area of an electron emission region of the electron emitting device 110 increases.